The present invention relates to a four wheel truck for a railway freight car and the like.
Modern freight car trucks are plagued by a wide variety of problems, both operational and in the area of maintenance. Some of these include truck hunting, which results in rapid and excessive wheel flange wear; excessive rotational restraining forces or torques which contribute to flange wear, and to wear of existing car body and truck center plates; car body roll and rock, resulting in structural damage to truck bolsters and car bodies, increased wear on these components, and wheel lifting; and suspension characteristics which accentuate the general self-destruct nature of the truck while also inflicting damage on lading and track structures.
A rigid wheel-axle set, having conventional tapered, conical, or otherwise profiled wheels, when displaced laterally of the center line of the track, executes two simultaneous motions; first, the wheel set moves toward its equilibrium (centered) position under the influence of gravity, and secondly, the high side wheel, rolling on a larger diameter than the low side wheel, moves along the rail faster than its partner, causing a yaw motion in the wheel set. The mass of the wheelset in moving toward its equilibrium point acquires kinetic energy in the yaw mode if damping or energy dissipation is less than critical. In this event the wheelset is displaced to the opposite side of the equilibrium point and the entire system of motions is reversed. Given the proper set of conditions, this motion may become a sustained harmonic oscillation with the sinusoidal peaks being clipped by contact between the rail and the wheel flange.
When rigid wheelsets are coupled in a truck thru the media of side frames, or rigid truck frames, the hunting tendency is transmitted to the truck and causes an oscillatory yawing motion of the truck about its center of rotation, or swivel. Continuous or semi-continuous motion of this nature is transmitted to the car body which then yaws about its center of gravity, or about the center plate of its second truck if that truck is not hunting.
A rigid wheelset is incapable of free curving when the radius of curvature &lt;dt/4cs where:
d = Wheel diameter PA1 t = Wheel tread contact spacing PA1 c = Flangeway clearance PA1 s = Effective conicity of tread
For relatively common conditions of d=33 inches, t=59 inches, c=0.375 inches, and s=1:20, the value of R is approximately 2200 feet.
For curves having a radius less than 2200 feet under the above conditions, the lead wheelset in a truck is guided around the curve by contact of the flange and the rail head. Inasmuch as the variation in wheel diameters is inadequate for free curving, the wheelset assumes a yawed attitude with the inside wheel tending to lead.
When wheelsets are coupled by side frames the entire truck is forced to assume a yawed attitude during curving. The force required to rotate the truck is transmitted through the flange of the outside lead wheel while the trailing axle tends to assume a quasi-radial location with respect to the curve. The optimum position when negotiating a curve is achieved when each wheel assumes a normal to the curve.